Mass unbalance in rotating components such as wheels, crankshafts, gas turbine engine rotors, etc. can cause undesirable vibration. Such vibration may cause damage to the component and the surrounding structure. Mass unbalance may be corrected by redistributing the mass of the component so as to position the centre of mass at the axis of rotation.
The effects of unbalance become more pronounced at higher rotational speeds. Consequently, unbalance is a particular issue in the aerospace industry where components rotate at extremely high speeds. Furthermore, vibration levels in an aerospace gas turbine engine are not only important from an engine reliability viewpoint but also from a passenger comfort viewpoint.
An unbalanced component exerts a force on its bearings which is given by:F=Mrω2 where M is the mass of the rotating component; r is the radial offset of the mass from its centre of mass; and ω is the angular velocity.
For a gas turbine engine, the mass of the rotor is relatively high and the angular velocity is extremely high. Therefore, it can be seen that any radial offset of the mass from its centre of mass (i.e. unbalance) results in a large force being exerted on the bearings. Consequently, it is necessary to minimise the radial offset.
Typically a Balance Grade of G2 (ISO 1940-1) may be imposed on gas turbine engines. For a rotor rotating at 10000 rpm, this equates to a permissible mass offset of 2 μm. This tolerance is some 200 times more stringent than those applied to everyday applications, such as car wheel balancing (Balance Grade G40).
Gas turbine engine rotors are conventionally balanced using the weight variation present in a set of aerofoil blades. The blades are detachably mounted to a disc of the rotor via slots provided around the circumference of the disc. The location of the blades around the rotor's disc can be varied to correct not only the disc unbalance but also that of the set of blades, thus producing a balanced rotor. With this method, the blades are weighed, or moment weighed and then distributed around the disc in a pattern to either minimise the unbalance in the blade set, or to compensate for the unbalance in the assembly.
In contrast, a blisk comprises a disc with integrally formed blades. This provides a considerable weight saving over the above described rotor by removing the fixtures required to detachably mount the blades to the disc. A reduction in mass of between 20% and 60% can be achieved by using a blisk. The reduction in weight provides an increase in the thrust to weight ratio, which leads to increased fuel economy and associated reduction in running costs, or to an increased payload for the aircraft. As a result, blisks are becoming more prevalent. However, blisks are very complex and time consuming to produce, and consequently, the cost per component is very high. Owing to the complexity of the manufacturing process, there is significant potential for non-conformance in the finished component, and the cost of rejection will again be very high. Furthermore, it is not possible to balance a blisk by interchanging blades and therefore it is necessary to correct the balance of the blisk using alternative methods.
One method of balancing a blisk is to bolt balancing weights onto the blisk to adjust its balance. However, adding weights to balance a blisk is counterproductive since the purpose of a blisk is to save weight. Furthermore, the weights increase the centrifugal loading on the blisk and can only be located in low stress areas. In addition, the connection between the weight and the blisk provides an interface where vibration may occur. This can lead to fretting and erosion. Also, the weights present a potential cause of Domestic Object Damage (DOD) to the engine, if the connection between a weight and the blisk fails.
An alternative method uses sacrificial balancing lands which are specifically provided on the component. These balancing lands may be machined to remove some or all of their mass and thus adjust the balance of the component. Such balancing methods require the provision of balancing lands or other features which can be later removed, if required. This results in the component being heavier than would otherwise be necessary.
Conventionally, the required size of the sacrificial balancing lands is calculated based on a worst case scenario for the distribution of the blades. This is where a 180 degree arc of maximum mass blades is located opposite a 180 degree arc of minimum mass blades. Although this ensures that all of the blisks produced can be balanced, the size of the balancing lands is excessive and counteracts the weight saving associated with a blisk.
The present invention seeks to provide a method for predicting initial unbalance in a blisk which provides a value of maximum probable unbalance rather than maximum possible unbalance, as is the case with the prior art method.